This invention relates to a multistage spray drying method for a detergent slurry, and particularly it relates to a multistage spray drying method for a detergent slurry comprising a relatively high content of a surfactant as active ingredient and a low content of a phosphate builder.
For obtaining a granular detergent from a detergent slurry, the spray drying method is generally used. In this method a detergent slurry is continuously spray-dried within a drying chamber wherein the temperature is maintained above 70.degree. C. Two types of spray drying methods are known. The first has a single spraying stage in which the spray nozzle is positioned in the top of the drying chamber, and the second has plural spraying stages. The single stage spray drying method has been generally used for the spray drying of a detergent slurry containing relatively high content of phosphate builder, and for drying a detergent slurry of this kind, it is possible to obtain a suitable granular detergent. However, in the case of spray drying a detergent slurry containing a low content of phosphate builder, mass production of a granular detergent having a desirable property is difficult by the single stage spray drying method.
Generally speaking, in the detergent manufacturing industry, to meet the restriction on the use of tripolyphosphate as well as the demand for economy of energy and resources, strenuous efforts are being directed to the development of a detergent which can minimize the standard amount of tripolyphosphate that is used. As a suitable detergent, the so-called compact low-phosphate detergent containing a relatively high content of surfactant and a low content of phosphate is being studied. However, in the case of manufacturing said compact low-phosphate detergent by the aforesaid single stage spray drying method, unless the amount of the slurry to be sprayed per unit time is remarkably decreased, it is difficult to produce a granular detergent having the necessary properties for a granular detergent. Under such circumstances, for the purpose of manufacturing a compact low-phosphate detergent, the foregoing multistage spray drying method may be preferably used.
In Japanese Pat. No. 3787/1972 there is disclosed a multistage spray drying method for a detergent slurry. In this method, the lowest level of the spray nozzles is located at below the isotherm of 88.degree. C. and above the isotherm of boiling point of the slurry. From 30 to 80% of the detergent slurry is sprayed from this lowest spraying stage, and the remainder is sprayed from the stage(s) disposed above the isotherm of 88.degree. C. According to the disclosure, by this multistage spray drying method, degradation of tripolyphosphate in spray drying can be minimized and fine powder and by-production of coarse powder can be decreased whereby a granular detergent of uniform size can be manufactured. This Japanese Pat. No. 3787/1972 further teaches that from the detergent slurry prepared by replacing a part or the entirety of tripolyphosphate with another builder, it is possible to obtain a satisfactory granular detergent by this multistage spray drying method. However, the result of a follow-up test of said multistage spray drying method actually conducted by the use of a detergent slurry containing a low-content of tripolyphosphate, for example, a detergent slurry containing less than 25% by weight of tripolyphosphate (dry basis), shows that as long as the ordinary amount of slurry per unit time is sprayed, it is impossible to obtain a granular detergent having scarcely agglomerated granules, and even when a lower amount of slurry per unit time is sprayed, the dry granules of the resulting detergent are apt to cake easily under dead load. Furthermore they tend to get pulverized with the passing of time.